As semiconductor device densities continue to get larger and isolation structures between devices continue to get smaller, the challenge of isolating individual devices from one another gets ever more difficult. Improper device isolation is the root cause of a number of device defects, including current leakages that waste power, latch-up that can cause intermittent (and sometimes permanent) damage to circuit functioning, noise margin degradation, voltage shift, and signal crosstalk, to name just some of the problems.
Prior device isolation techniques included local oxidization on silicon (LOCOS) processes that laterally isolated the active device regions on the semiconductor device. LOCOS processes, however, have some well known shortcomings: Lateral oxidization of silicon underneath a silicon nitride mask make the edge of field oxide resemble the shape of a “bird's beak.” In addition, lateral diffusion of channel-stop dopants make the dopants encroach into the active device regions, thereby overshrinking the width of the channel region. These and other problems with LOCOS processes were exacerbated as device size continued to shrink with very large scale integration (VLSI) implementation, and new isolation techniques were needed.
Current isolation techniques include shallow trench isolation (STI) processes. Early STI processes typically included etching a trench having a predetermined width and depth into a silicon substrate, filling the trench with a layer of dielectric material (e.g., silicon dioxide), and finally planarizing the dielectric materials by, for example, chemical-mechanical polishing (CMP). For a time, the early STI processes were effective for isolating devices spaced closer together (e.g., 150 nm or more), but as the inter-device space continued to shrink, problems developed.
One of these problems is avoiding the formation of voids and weak seams during the deposition of dielectric material in the trenches. As trench widths continue to shrink, the aspect ratio of trench height to trench width gets higher, and high-aspect ratio trenches (e.g., aspect ratios of about 6:1 or more) are more prone to form voids in the dielectric material due to the premature closure of the trench (e.g., the “bread-loafing” of the dielectric material around the top corners of the trench). The weak seams and voids create uneven regions of dielectric characteristics in trench isolations, which adversely impact the electrical characteristics of the adjacent devices and can even result in device failure.
One technique for avoiding voids is to reduce the deposition rate to a point were the dielectric material evenly fills the trench from the bottom up. While this technique has shown some effectiveness, it slows the overall production time and thereby reduces production efficiency. Thus, there remains a need for device isolation techniques that include the efficient filling of inter-device trenches that also reduce and/or eliminate voids created in the filled trenches.